Method of manufacturing semi-



United States PatentO METHOD OF MANUFACTURING SEMI- CONDUCTIVE MATERIAL Pieter Willem Haayman, Arie Bol, Frans Cornelis Romeyn, and Evert Johannes Willem Verwey, Eindhoven, Netherlands, assignors to Hartford National Bank and Trust Company, Hartford, Conn., as trustee No Drawing. Application December 19, 1947,

- Serial No. 792,850

Claims priority, application Netherlands January 8, 1947 1 Claim. 31. 252-419) occurrence of the metal ion concerned in different va-- lencies, so that conducting electrons may travel via these ions through the crystal lattice.

In practice it is found, however, that it is very difficult to regulate the oxygen content in a manner sufliciently reproducible and accurate by the choice of the temperature and duration of the heating process and the partial pressure of the oxygen contained in the surrounding gaseous atmosphere. Furthermore, the stability of the materials obtained, more particularly at high temperatures, is frequently insuflicient so that such materials can be used only in a limited range of temperatures. It may be assumed that the low stability of such materials is connected with the production of open lattice positions or the filling-up of interstitial lattice positions as a result of the stoichiometric deviation.

The said disadvantages occur to an appreciable smaller extent if for the conduction use is made of compounds containing the metal ion in different valencies without lattice divergences occurring, such as FeaO4. The improvement is particularly important if such compounds are used in the form of mixed crystals with compounds which are in themselves highly stable and non-conductive, even at comparatively high temperatures. In the case of Fe3O4 use is made for this purpose of double oxides such as MgO.Al2O3 and 2NiO.SnOz, which like FeaO4 exhibit spinel structure.

According to the copending U. S. patent application Serial No. 766,876, filed August 6, 1947, and now abandoned, in materials exhibiting a divergence in the stoi chiometric composition the said disadvantages may be counteracted by the incorporation of other ions in the lattice in such a manner that the metal ion of the basal material occurs in ditferent valencies without involving, however, open positions in the lattice or occupies interstitial lattice positions, which may result in decreased stability.

As described in the said patent application, this result may be obtained in that a metal compound comprising a metal ion which may occur in more than one valency is heated jointly with a metal compound of the same metalloid but comprising a metal ion having a valency ditiering from that of the first metal ion so as to produce mixed crystals in the lattice of which the metal ions of the substance added occupy lattice positions of the other metal ions and a number of the latter correspondingly change their valency. Thus, for example, it is pos- 2,735,824 Patented Feb. 21, 1956 sible to obtain a nickel oxide lattice comprising simultaneously bivalent and trivalent nickel the open positions of which have been filled up by incorporating lithium ions. This may be carried out, for example, by heating NiO with LiCOs in a suitable mixing ratio.

It has been found that in the case of iron oxide, which is very attractive in the mass production of resistances on account of its low cost as raw material, although this metal does not exhibit lattice deviations in the form in which it is conductive, namely as Fe3O4, it is nevertheless of importance that the principle described in the copending U. S. Patent application Serial No. 766,876 should be utilised. It is thus readily possible to obtain semi-conductive material which is very well reproducible as far as its resistanceproperties are concerned and highly stable in air even at high temperatures.

According to the invention, which is based on this principle, semi-conductive material on the basis of iron oxide is obtained by heating a mixture consisting of the oxide of trivalent iron and an oxide of a metal of different valency which, upon heating with the oxide of bivalent iron, may result in a compound yielding a homogeneous mixed crystal with the oxide of trivalent iron.

In carrying out the process it is not necessary to utilise the two oxides as such. It is alternatively possible, instead of using one of the said oxides, to utilise a compound which upon heating is converted into the corresponding oxide.

Among the compounds to be admixed with the iron oxide particular mention may be made of TiOz, ZrOz and SnOz in a quantity chosen in accordance with the desired number of bivalent iron ions in the final product, which is determinative of the resistance properties.

In the case of simple sintering together in air at a temperature of 1200 C. a portion of the F6203 reacts with the compound added, for example TiOz, while splitting off oxygen, in which event a product is obtained which comprises Fe++, Fe+++ and Ti++++ and which might be regarded as to be composed of mixed crystals of the compounds FezOs and FeTiOs (ilmenite), which are not conductive in themselves. At the same conditions pure FezOs does not yet split off oxygen and hence remains an insulator.

The procedure adopted may be, for example, as follows:

99 mol. percent of FezOa and 1 mol. per cent of TiOz are ground in a ball mill with alcohol for 4 hours. After drying the pulverulent mixture obtained is plasticised with a binder and Water and formed into rods of 6.5 mm. in diameter by means of an extrusion press. Subsequently, the rods are divided into lengths of 40 mm., followed by heating in air at 1200 C. for 2 hours. Finally, contact caps are pressed on the resistance rods. The resistance value at 20 C. is 2100 ohms, at 50 C. 870 ohms, and at C. 212 ohms.

Upon titration of the product obtained it appeared that the small admixture of 1 mol. percent of T iOz had actually given rise to the forming of 1 mol. percent of bivalent iron oxide.

Furthermore it may be observed in this connection that the rate of cooling subsequent to the sintering process is entirely without influence on the resistance properties, which did not change at the least even when the material was allowed to cool down simultaneously with the furnace in which sintering had taken place and hence at a very low rate. This shows that the products obtained are stable in air at any temperature up to the sintering temperature and hence may be loaded up-to this temperature without any risk.

The desired specific resistance and temperature coefiicient may be accurately adustjed by a proper choice of the amount of the compound liable to the forming of the bivalent iron in addition to the trivalent iron. However, if a very high specific resistance and a highly negative temperature coefficient are desired, this may lead to practical difiiculties since the very small quantities then to be used cannot in a simple manner be distributed entirely homogeneously in the iron oxide.

In view thereof it is advantageous that the resistance properties may be varied in addition by the admixture of non-conductive oxides which, together with FezOa, yield mixed crystals such as CrzOs. If in the mixture according to the above-mentioned example 15 mol. percent of F 20 are substituted by CrzOs and the mass is Worked up in a similar manner, the resistance value with equal proportioning of the resistances is 20,000 ohms at 20 C., 6-560 ohms at 50 C. and 1030 ohms at 120 C.

With the above-mentioned ways of adjusting the resistance properties steps can always be taken to ensure a final product which wholly consists of one phase (mixed crystal). The resistance properties can, however, also be varied by adding to the initial mixture an insulating material such as a ceramic, for example, bentonite, which results in the formation of a second phase. If, for example, an amount of by weight of bentonite is added to the above-mentioned mixture consisting of 99 mol. percent of FezOs and 1 mol. percent of T and this mixture is worked up in a similar manner as mentioned above, one obtains resistances having a value of 60,000

ohms at 20 C., of 15,600 ohms at C., and of 1400 ohms at C.

The resistances of the non-conductive material according to the invention are suitable, for example, for eliminating voltage pulses, compensation of the temperature coefficient of metal-wire resistances and measurements of temperature.

What we claim is:

A resistor having a given value of specific resistance consisting essentially of substantially homogeneous mixed crystals produced by heating a mixture of about 99 mol. percent of ferric oxide and about 1 mo percent of titanium dioxide in air at a temperature of about 1200" C.

References Cited in the file of this patent OTHER REFERENCES Zeit fur Anorg und All Chemie, vol. 149, page 73, Tamman (1925).

Mellor: Treatise on Inorganic and Theoretical Chemistry, v. 13, page 815 (1934). 

